Colloid Transport in Aggregated Porous Media with Intra- and Interaggregate Porosities

Column tracer and colloid transport experiments were performed under both saturated and unsaturated steady state flow conditions in an aggregated porous medium with a bimodal pore size distribution (PSD): intra-aggregate porosity with a pore radius between 10(-2 )and 10(-1) mu m and interaggregate porosity ranged between 10(1) and 10(3) mu m (interporosity). All experiments were carried out under unfavorable conditions for physicochemical attachment to solid-water interfaces, using negatively charged porous media and latex microspheres (1 mu m). Both porous media and colloids used in this work were hydrophobic. The results obtained through experimental observations and numerical simulations in the aggregated medium were confronted with those obtained for a sandy medium, characterized by a narrow unimodal PSD with a pore radius which ranged between 10(1) and 10(2) mu m (interporosity), to explore the relative importance of the PSD on water flow, colloid transport, and deposition. Physicochemical interactions between colloids and porous media, calculated according to the Derjaguin- Landau-Verwey-Overbeek (DLVO) theory, showed no primary minimum and low secondary minimum depth, suggesting reversible colloid retention and possibility for colloid detachment by hydrodynamics drag for both sand and aggregated media. Hydrodynamic drag forces were slightly greater than the resisting adhesive DLVO forces in the secondary minimum, indicating a possibility for colloid detachment. For the same flow rate, more nonuniform transport of colloids were obtained in the bimodal aggregated medium compared to the unimodal sand. If the nonuniform and preferential transport of colloids should contribute in decreasing of colloids retention, particularly under saturated flow conditions, surprisingly greater overall colloid retention was obtained in the aggregated medium. Based on the PSD, colloid exclusion from smaller pores has a higher probability to occur in the aggregated medium compared to the sand. Therefore, size exclusion contributed to the overall preferential transport in this dual porosity medium, and it was expected that this nonuniform transport would disfavor colloid retention. However, colloid retention efficiency at the column scale was higher for the aggregated medium compared to the sand under saturated flow conditions, despite a more uniform flow and transport in the later. This means that the presence of the intraporosity in the aggregated medium and related small pores contributed not only in colloid size exclusion, but it had also an opposite effect, resulting in additional deposition sites of colloid particles. Under unsaturated flow conditions, capillary forces governed colloid retention, independently from the PSD.